The present invention relates to an electrophotographic apparatus and a method of forming an image on a recording medium, particularly, to a wet electrophotographic apparatus and a method of forming a image ion a recording medium using a liquid developer.
In a wet electrophotographic technology, a liquid developer prepared by dispersing toner in a petroleum solvent is used, and an electrophoresis of the toner within the petroleum solvent is utilized in the developing process. The wet electrophotographic technology produces various merits that cannot be achieved in the dry electrophotographic technology and, thus, attracts attentions in this technical field in recent years.
For example, the wet electrophotographic technology permits using a very fine toner having a particle diameter of sub-micron order so as to make it possible to achieve a high image quality. Also, since it is possible to obtain a sufficiently high image density with a small amount of the toner, the wet electrophotographic technology is advantageous in economy. In addition, the wet electrophotographic technology permits realizing a texture fully comparable with that of the printing, e.g., an offset printing. Further, since the toner can be fixed to a paper sheet under a relatively low temperature, the energy saving can be achieved in the wet electrophotographic technology.
FIG. 1 schematically shows a conventional wet electrophotographic apparatus. In the conventional wet electrophotographic apparatus shown in FIG. 1, the image formation is performed as follows. In the first step, the image holding surface of a photoconductor drum 101, which is an electrophotographic photoconductor, is uniformly charged by a charger 102-1 while rotating the photoconductor drum 101 in the clockwise direction. Then, the charged image holding surface is exposed to a laser beam 103-1 modulated to conform with the image information of yellow. As a result, an electrostatic latent image conforming with the yellow image is formed on the image holding surface. Then, a yellow liquid developer is supplied from a developing device 104-1 onto the image holding surface having the electrostatic latent image formed thereon. As a result, a yellow developer image conforming with the electrostatic latent image is formed on the image holding surface.
The image holding surface having the yellow developer image formed thereon is charged by a charger 102-2 and, then, is exposed to a laser beam 103-2. Further, a magenta liquid developer is supplied from a developing device 104-2 onto the image holding surface. As a result, a magenta developer image is formed on the image holding surface in addition to the yellow developer image. Further, the image holding surface having the yellow and magenta developer images formed thereon is charged by a charger 102-3 and, then, is exposed to a laser beam 103-3. Still further, a cyan liquid developer is supplied from a developing device 104-3 onto the image holding surface. In the next step, the image holding surface is charged by a charger 102-4 and, then, exposed to a laser beam 103-4. Further, a black liquid developer is supplied from a developing device 104-4 onto the image holding surface. As a result, yellow, magenta, cyan and black developer images are formed on the image holding surface of the photoconductor drum 101.
The developer images of these four colors are transferred from the image holding surface onto an intermediate transfer roller 105 and, then, are further transferred from the intermediate transfer roller 105 onto a paper sheet 106. A pressure transfer system in which pressure is applied from a press roller 107 onto the intermediate transfer roller 105 via the paper sheet 106 is employed in the transfer of the developer image from the intermediate transfer roller 105 onto the paper sheet 106. The pressure transfer system is also employed for the transfer of the developer image from the photoconductor drum 101 onto the intermediate transfer roller 105. In this case, however, a contact pressure is applied from the intermediate transfer roller 105 to the photoconductor drum 101, and the intermediate transfer roller 105 is heated. Incidentally, the transfer system in which the intermediate transfer roller 105 is heated is disclosed in U.S. Pat. No. 5,570,173, the entire contents of which are incorporated herein by reference.
The present inventors have found that, in the wet electrophotographic apparatus shown in FIG. 1, a very high transfer efficiency can be achieved in the case where the relationship Td less than Tg less than Tr is satisfied among the temperature Td of the image holding surface of the photoconductor drum 101, the surface temperature Tr of the intermediate transfer roller 105 and the softening temperature (glass transition temperature) Tg of the toner contained in the liquid developer. Note that the term xe2x80x9csoftening temperaturexe2x80x9d denotes the temperature dividing the degree of fluidity of a material. To be more specific, under temperatures lower than the softening temperature, the plastic viscosity of the material is markedly low. However, under temperatures equal to or higher than the softening temperature, the material exhibits a prominent fluidity and is soft.
For example, in the case of using a liquid developer containing a toner having a softening temperature Tg of 40xc2x0 C., a very high transfer efficiency can be achieved, if the temperature Td of the image holding surface of the photoconductor drum 101 is lower than 40xc2x0 C. and the surface temperature Tr of the intermediate transfer roller 105 is higher than 40xc2x0 C. The particular relationship can be achieved in general by heating the intermediate transfer roller 105 by the heater in the intermediate transfer roller 105.
However, in the conventional wet electrophotographic apparatus shown in FIG. 1, it is impossible to avoid the heat conduction from the intermediate transfer roller to the photoconductor drum 101, with the result that the heat is accumulated in the photoconductor drum 101. What should be noted is that, where the conventional wet electrophotographic apparatus having such a heater is used over a long period of time, it is possible for the temperature Td of the image holding surface of the photoconductor drum 101 to be elevated to a level substantially equal to the surface temperature Tr of the intermediate transfer roller 105. For example, where the temperature Tr is about 50xc2x0 C. to 80xc2x0 C., the temperature Td is considered to be elevated to reach about 40xc2x0 C. to 70xc2x0 C.
Where the image holding surface of the photoconductor drum 101 is heated to a high temperature, it is impossible to satisfy the particular relationship denoted by the inequality noted above, leading to a problem that the transfer efficiency of the developer image from the photoconductor drum 101 to the intermediate transfer roller 105 is markedly impaired.
What should also be noted is that, where the image holding surface of the photoconductor drum 101 is heated to a high temperature, the liquid developers in the developing devices 104-1 to 104-4 are also heated so as to evaporate the carrier solvent contained in the liquid developer. Since it is undesirable in terms of the environmental problem for the evaporated solvent to leak to the outside of the case of the electrophotographic apparatus, it is necessary to further arrange a special means for preventing the evaporated solvent from leaking to the outside of the electrophotographic apparatus. Also, in this case, an additional problem is generated that the cycle for replenishing the solvent is shortened.
In order to cope with these problems, it is conceivable to arrange, for example, a water-cooling type or an air-cooling type cooling mechanism inside the photoconductor drum 101 so as to maintain constant the temperature of the image holding surface. However, in the case of arranging a cooling mechanism inside the photoconductor drum 101, the electrophotographic apparatus is rendered bulky and complex in construction.
Also, where a water-cooling type cooling mechanism is arranged inside the photoconductor drum 101, it is necessary to recover and renew the cooling water periodically, with the result that much labor is required for the maintenance of the electrophotographic apparatus.
An object of the present invention is to provide a wet electrophotographic apparatus and a method of forming an image on a recording medium capable of maintaining a high transfer efficiency even in the case where the apparatus is operated over a long time.
Another object of the present invention is to provide a wet electrophotographic apparatus and a method of forming an image on a recording medium capable of suppressing the evaporation of the carrier solvent contained in the liquid developer.
Still another object of the present invention is to provide a wet electrophotographic apparatus and a method of forming an image on a recording medium capable of cooling the image holding surface of the electrophotographic photoconductor and also capable of saving the labor required for the maintenance.
According to a first aspect of the present invention, there is provided a wet electrophotographic apparatus forming an image on a recording medium comprising an electrophotographic photoconductor having an image holding surface; a latent image forming unit configured to form an electrostatic latent image on the image holding surface; a developing unit configured to form a developer image on the image holding surface having the latent image formed thereon by using a liquid developer containing a carrier solvent and a toner dispersed in the carrier solvent, the developing unit comprising a reservoir configured to reserve the liquid developer, a container connected to the reservoir and configured to be supplied with the liquid developer from the reservoir, and a developer feeding surface arranged to be in contact with the liquid developer in the container and configured to move near the image holding surface with the liquid developer interposed therebetween so as to supply the liquid developer in the container onto the image holding surface; a transfer unit configured to transfer the developer image from the image holding surface onto the recording medium; a first temperature measuring device configured to measure a temperature of the image holding surface; a cooler configured to cool the liquid developer in the reservoir; and a controller connected to the first temperature measuring device and to the cooler and configured to compare a first temperature measured by the first temperature measuring device with a first set value which is equal to or lower than a softening temperature of the toner and control an operation of the cooler to lower the first temperature in a case where the first temperature is equal to or higher than the first set value.
According to a second aspect of the present invention, there is provided a wet electrophotographic apparatus forming an image on a recording medium, comprising an electrophotographic photoconductor having an image holding surface; a latent image forming unit configured to form an electrostatic latent image on the image holding surface; a developing unit configured to form a developer image on the image holding surface having the latent image formed thereon by using a liquid developer containing a carrier solvent and a toner dispersed in the carrier solvent, the developing unit comprising a reservoir configured to reserve the liquid developer, a container connected to the reservoir and configured to be supplied with the liquid developer from the reservoir, and a developer feeding surface arranged to be in contact with the liquid developer in the container and configured to move near the image holding surface with the liquid developer interposed therebetween so as to supply the liquid developer in the container onto the image holding surface; a transfer unit configured to transfer the developer image from the image holding surface onto the recording medium; a first temperature measuring device configured to measure a temperature of the liquid developer in the reservoir; a cooler configured to cool the liquid developer in the reservoir; and a controller connected to the first temperature measuring device and to the cooler and configured to compare a first temperature measured by the first temperature measuring device with a first set value which is equal to or lower than a softening temperature of the toner and control an operation of the cooler to lower the first temperature in a case where the first temperature is equal to or higher than the first set value.
According to a third aspect of the present invention, there is provided a method of forming an image on a recording medium comprising forming a latent image on a image holding surface of a electrophotographic photoconductor; forming a developer image on the image holding surface having the latent image thereon by supplying the image holding surface with a liquid developer containing a carrier solvent and a toner dispersed in the carrier solvent; transferring the developer image from the image holding surface onto the recording medium; setting a set value which is equal to or lower than a softening temperature of the toner; measuring a temperature of the image holding surface; comparing the temperature of the image holding surface measured with the set value; and cooling the liquid developer before supplying the image holding surface with the liquid developer in a case where the temperature of the image holding surface measured is equal to or higher than the set value.
According to a fourth aspect of the present invention, there is provided a method of forming an image on a recording medium comprising forming a latent image on a image holding surface of a electrophotographic photoconductor; forming a developer image on the image holding surface having the latent image thereon by supplying the image holding surface with a liquid developer containing a carrier solvent and a toner dispersed in the carrier solvent; transferring the developer image from the image holding surface onto the recording medium; setting a set value which is equal to or lower than a softening temperature of the toner; measuring a temperature of the liquid developer before supplying the image holding surface with the liquid developer; comparing the temperature of the liquid developer measured with the set value; and cooling the liquid developer before supplying the image holding surface with the liquid developer in a case where the temperature of the liquid developer measured is equal to or higher than the set value.
As described above, the liquid developer in the reservoir is cooled according to the first to forth aspects of the present invention. Where the liquid developer is cooled, it is possible to utilize the liquid developer as a coolant for cooling the image holding surface. Therefore, according to the first to forth aspects of the present invention, it is possible to maintain a high transfer efficiency even if the apparatus is continuously operated for a long time. Also, since the liquid developer is cooled, it is possible to prevent the toner deterioration and to suppress the evaporation of the carrier solvent. Further, since the liquid developer, which is a consumption article, is utilized as a coolant for cooling the image holding surface, the image holding surface can be cooled without requiring a much labor for the maintenance.
In the first to forth aspects of the present invention, it is desirable for the transfer unit to include an intermediate transferring member arranged near the electrophotographic photoconductor and configured to transfer the developer image from the image holding surface onto the recording medium. It is also desirable for the transfer unit to include a heater configured to heating the surface of the intermediate transferring member to a temperature higher than the softening temperature of the toner. The first to forth aspects of present invention are particularly effective in such a case.
In the first to forth aspects of the present invention, it is possible for the controller to be configured to compare the temperature of the image holding surface measured by the temperature measuring device with a set value which is, e.g., equal to or higher than the softening temperature noted above and control the operation of the apparatus to halt the printing in the case where the temperature of the image holding surface is higher than the set value.
In the first to forth aspects of the present invention, it is desirable for the controller to be configured to compare the temperature of the image holding surface measured by the temperature measuring device with a set value which is equal to or lower than the softening temperature of the toner and control the operation of the cooler to lower the temperature of the image holding surface. In this case, it is possible to achieve a particularly high transfer efficiency.
Where a temperature measuring device configured to measure the temperature of the liquid developer in the reservoir is arranged in the wet electrophotographic apparatus, it is possible to perform various controls based on the temperature of the liquid developer to be measured. For example, it is possible to set in advance a set value substantially equal to or higher than the softening temperature noted above in the controller. In this case, it is possible to judge whether or not it is necessary to renew the liquid developer by comparing the temperature of the liquid developer in the reservoir with the set value. It is also possible to set in advance a set value equal to or lower than the softening temperature noted above in the controller. In this case, it is possible to control the operation of the cooler based on the result obtained by comparing the temperature of the liquid developer in the reservoir with the set value. Where the operation of the cooler are controlled in this fashion, it is possible to make the temperature of the image holding surface lower than the softening temperature noted above. It is also possible to prevent deterioration of the liquid developer.
It is desirable for the wet electrophotographic apparatus according to the first to forth aspects of the present invention to further comprises a case configured to cover the above-noted constituents and a temperature measuring device arranged within the case which measures the temperature within the case. In this case, it is desirable to control the operation of the cooler such that the temperature of the liquid developer in the reservoir is rendered equal to or higher than a value which is equal to or calculated from the temperature within the case. By controlling the operation of the cooler in this fashion, it is possible to prevent the vapor within the case from being condensed into dew on the developing unit.